I pull a 10k trailer with the wifes 2016 1500 chevy with built in brake control. I've found 3 to 3.5 gain works great. More gain = brake lockup and flat spotting the tires.
On all my new or used trailers I remove all clam shell type wire connectors and solder those connections. I also add some type of sealant like a 35 year siliconized caulk or a pure silicone/etc in the hole around wiring where it goes into the axle. I then check to make sure their getting proper amps.
I've had trailers that had 12v at the magnets but very little amps....and had poor brakes.